1. Field of the Invention
The invention relates to an additive combination of (a) an ethylene backbone middle distillate fuel oil pour depressant polymer with (b) a second polymer having alkyl side chains of 6 to 18 carbon atoms defined by dicarboxylic acid ester or olefin moieties. This combination is particularly useful in middle distillate fuel oils containing a fraction boiling above 700.degree. F., for controlling the size of wax crystals that form at low temperatures.
2. Description of the Prior Art
Various polymers, useful as middle distillate pour point depressants, prepared from ethylene have been described in the patent literature. These pour depressants include copolymers of ethylene and vinyl esters of lower fatty acids such as vinyl acetate (U.S. Pat. No. 3,048,479); copolymers of ethylene and alkyl acrylate (Canadian Patent No. 676,875); terpolymers of ethylene with vinyl esters and alkyl fumarates (U.S. Pat. Nos. 3,304,261 and 3,341,309); polymers of ethylene with other lower olefins, or homopolymers of ethylene (British Pat. Nos. 848,777 and 993,744); chlorinated polyethylene (Belgian Pat. No. 707,371 and U.S. Pat. No. 3,337,313); etc.
Polymers having alkyl groups in the range of C.sub.6 to C.sub.18, such as homopolymers and copolymers of olefins; alkyl esters of unsaturated dicarboxylic acids (e.g., copolymers of dialkyl fumarate with vinyl acetate) and copolymers of olefins and said esters, are known in the art principally as lube oil pour depressants and/or V.I. improvers. For example, U.S. Pat. No. 2,379,728 teaches olefin polymers as lube pour depressants; U.S. Pat. No. 2,460,035 shows polyfumarates; U.S. Pat. No. 2,936,300 shows a copolymer of dialkyl fumarate and vinyl acetate; while U.S. Pat. No. 2,542,542 teaches copolymers of olefins, such as octadecene with maleic anhydride esterified with alcohol, e.g., lauryl alcohol, in lube and heating oils.
Synergistic pour point depressing combinations of various members of the above-noted two types of polymers in heavy fuels, e.g., residua and flash distillate fuels, which fuels contain relatively large amounts of waxes having 20 or more carbon atoms, is taught in U.S. Pat. No. 3,726,653.